Visuals (listed by section)

Figure 6-1. Chemical Formula for Photosynthesis

Photosynthesis is the process by which plants turn carbon dioxide from the atmosphere into carbohydrates (plant tissue), using light as an energy source and releasing oxygen as a byproduct. Photosynthesis is one of the most important biological processes on Earth: It makes air breathable, and supplies most of the energy required for all life forms. (Unit 6, Section 1)

Figure 6-2. Combustion Analysis Setup

In a combustion analysis, burning a test compound breaks it into its component elements, which combine with oxygen from the surrounding air to produce various gases. Here, carbon in the material combines with oxygen to make carbon dioxide (CO2), and hydrogen combines with oxygen to make water vapor (H2O). These gases are collected by passing them through substances that trap each gas product separately. By weighing these absorbers, researchers can calculate how much of each gas was produced, and then deduce the empirical formula of the test compound. (Unit 6, Section 2)

Figure 6-3. Amedeo Avogadro (1776–1856)

This great Italian scientist made important contributions to molecular theory, particularly Avogadro's Law. He began as a lawyer, and his interest in math and physics led him to become the first physics professor in Italy in 1820. It is in his honor that the number of objects in a mole is called "Avogadro's Number." (Unit 6, Section 3)

Sidebar Image. Avogadro's Law Example

Equal volumes of any gasses contain equal numbers of particles (Avogadro's rule). If both balloons are in the same room and we assume that the volume in the balloon with helium on the left is equal to the volume of the balloon with carbon dioxide on the right, the amount of gas in each balloon is equal. (Unit 6, Section 3)

Figure 6-4. Finding the Atomic Mass on the Periodic Table of Elements

On the periodic table, the elements are ordered by their atomic numbers, which is the number of protons that particular atom has in the nucleus. Atomic mass is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally occurring element. In the case of helium, it is designated as 4.003 and is shown in the bottom right-hand corner. (Unit 6, Section 4)

Sidebar Image. Plastic Sulfur

Figure 6-5. Mass Percentage of Aluminum in Aluminum Hydroxide

The molecular formula gives us the number of each type of atom in a molecule, but we can also discuss the composition of a molecule in terms of how much of its mass is contributed by each atom type. We call this the "mass percentage" of a given atom type in a particular compound. (Unit 6, Section 5)

Figure 6-6. Mass Percentage of Salt in Seawater

Sea salt can be easily made by letting artificial ponds like these fill with seawater at high tide, then letting the water evaporate to leave the ionic compounds behind. The salt prepared this way is mostly sodium chloride, with a wide variety of other ions in smaller amounts (sulfate, magnesium, potassium, calcium, and many more). Sea salt is considered to be more of a delicacy when cooking as many chefs claim it tastes better. It is also more expensive to produce than normal table salt, and lacks the iodide that is added to normal table salt to prevent thyroid conditions caused by iodide deficiency. (Unit 6, Section 5)

Figure 6-8. Reaction of Methane Combustion

In this combustion reaction, methane combines with oxygen to make water and carbon dioxide. Looking at an inventory of the atoms, note how the number of atoms of each type on each side of the equation match perfectly. This is because this equation is balanced. In order to balance this equation, it took two oxygen molecules and two water molecules, which is why both of those chemicals have coefficients of two in the balanced reaction. (Unit 6, Section 6)

Figure 6-9. The Dissociation of Lead (II) Sulfate into Its Constituent Ions

The net charge (or total charge) must be equal on both sides of a balanced equation. The charge for lead sulfate on the left is zero. Looking at the products of the reaction, the 2+ and 2- charges for its constituent ions on the right added together also equal zero. Therefore, the charges are balanced. (Unit 6, Section 6)

Figure 6-7. Writing Chemical Equations

Methane (CH4) and dioxygen (O2) react to make water (H2O) and carbon dioxide (CO2). The Lewis structure for each molecule is on top, and the molecular formula is on the bottom. One carbon atom, three oxygen atoms, and two hydrogen atoms yield one carbon atom, three oxygen atoms, and two hydrogen atoms. (Unit 6, Section 6)

Figure 6-10. Limewater

The image on the left shows the production of limewater by creating a supersaturated solution of calcium hydroxide. Limewater is an important ingredient in the preparation of corn tortillas. The image on the right is a lithograph of women making tortillas. (Unit 6, Section 7)

Figure 6-11. Exploring the Stoichiometry of a Balanced Equation

Because one molecule of calcium hydroxide is combining with the carbonic acid, and two molecules of water are being produced, we write the number two in front of the water to balance the equation. This means, in words, that one mole of calcium hydroxide reacts with one mole of carbonic acid to produce one mole of calcium carbonate and two moles of water. (Unit 6, Section 7)

Figure 6-13. Recognizing Limiting Reagents

The top equation is a balanced equation showing the combustion of methane in the presence of oxygen. The first line shows the two starting reactants, and points out that the oxygen is the limiting reagent. Using the stoichiometric ratios along with the amount of oxygen present, the amount of substances used up and created by this reaction can be found. At the bottom of the figure, there is an inventory of what is left after the reaction is complete. Note that the only thing that is at zero is the limiting reagent that was completely used up. (Unit 6, Section 8)

Figure 6-12. Making Chocolate Chip Cookies

This photo shows the exact amount of each ingredient needed to make exactly one full batch of chocolate chip cookies: brown sugar, flour, chocolate chips, butter, eggs, and a splash of vanilla. When we check to see if we have the ingredients to make a full batch of cookies, if any of the ingredients are lower than the called-for amount, we'd have to scale down the recipe to fit that ingredient: The ingredient that is less than is required for a full batch of cookies is a limiting ingredient. In chemistry, an ingredient that is used up first during a reaction is called the "limiting reagent." (Unit 6, Section 8)

Figure 6-14. Finding the Theoretical Yield

To calculate the percent yield of a reaction, we determine the number of moles of product we would have if every molecule of the limiting reagent were converted to products; we call this the "theoretical yield." In this example, we are assuming that we have two moles (or 56 grams) of carbon monoxide with an unlimited amount of hydrogen gas. Therefore, the carbon monoxide is our limiting reagent and that's the amount in moles we will use to stoichiometrically determine the maximum amount of each product we can make. Thus, we can make two moles of either one of the two products because all the ratios are 1-to-1. When the reaction is done in the laboratory, we measure the actual yield and compare it to the theoretical yield to find the percent yield. (Unit 6, Section 9)

Figure 6-15. Artificial Leaf Using Light Energy to Split Water into Hydrogen and Oxygen ...

Professor Dan Nocera and his team at MIT are developing a technology in the lab that is similar to what happens in green plants: Energy from light is converted into chemical energy that can be stored as a fuel—in this case, hydrogen gas. The reaction can be written as 2H2O → 2H2 + O2. This balanced chemical equation can be used to predict how many grams of hydrogen fuel will result from the conversion of any given amount of water. (Unit 6, Section 10)